2017: fix exporting-go.pdf, slide 26

The readNode function must return a node.

Change-Id: I37926323c5e7d6887c950d948419913bd32d8547
Reviewed-on: https://go-review.googlesource.com/44990
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/2017/exporting-go.pdf b/2017/exporting-go.pdf
index febdd81..d8c0b53 100644
--- a/2017/exporting-go.pdf
+++ b/2017/exporting-go.pdf
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